1. Field of the Invention
The present invention relates to a band spread radio communication system, and more particularly to an automatic gain control method and device for use in a communication terminal of a mobile radio communication system employing a time-division duplex (TDD) transmission.
2. Description of the Related Art
In a conventional mobile radio communication system employing a time-division duplex (TDD) transmission, a mobile station automatically controls a gain of a transmitter/receiver in a communication terminal, in order to prevent a deterioration of the system performances due to a mutual interference with another mobile station. The time-division duplex transmission refers to a bidirectional transmission for periodically (e.g., 1msec) switching a channel to a transmission mode and a reception mode. A mobile station controls a gain of a transmission power amplifier disposed in a transmitter based on a mobile station transmission power control command included in a received frame from a base station, in order to maintain an appropriate transmission power during a transmission mode. Further, the mobile station appropriately controls a gain of a reception amplifier in order to compensate for an attenuation of a reception power according to a distance from the base station, during a reception mode.
Conventionally, the gains of the transmission power amplifier and the reception amplifier are controlled by an analog automatic gain control method in which the gain is controlled according to an intensity of a received signal. Such a prior art method is well disclosed in U.S. Pat. No. 5,257,283, issued on Oct. 26, 1993, by Klein S. Gilhousen, el al., entitled Spread Spectrilm Transismitter Power Control Method And System.
Referring to FIG. 1, which is described in the U.S. Pat. No. 5,257,283 patent, there is illustrated a block diagram of an analog automatic gain control system for use in a mobile radio communication system. As illustrated, a signal received from an antenna is transferred to an intermediate frequency amplifier (IF AMP) 6 via a down-converter 2 and a band pass filter (BPF) 4. An output of intermediate frequency amplifier 6 is applied to a base band processor 10 via an analog-to-digital (A/D) converter 8, and also applied to an automatic gain control (AGC) detector 12. Automatic gain control detector 12 has a double function. First, in the reception mode, automatic gain control detector 12 generates a control voltage for controlling a gain of intermediate frequency amplifier 6 to maintain a constant level of the received signal. Second, in the transmission mode, an output of automatic gain control detector 12 is compared with a reference power level value generated from a control processor 18, in order to control a transmission power based on a value set by a transmission signal from the base station. A comparator 14 compares the output of the automatic gain control detector 12 with the reference power level value received from control processor 18 via a digital-to-analog (D/A) converter 20. An output of comparator 14 is applied to a nonlinear filter 16 which has a nonlinear output characteristic with respect to a variation of the output of comparator 14. The output characteristic of nonlinear filter 16 sets an increasing rate of the transmission power control voltage to a value lower than a decreasing rate thereof, in order to prevent the deterioration of the system performances due to a rapid increase of the transmission power. In the meantime, an intermediate frequency amplifier 24 controls a signal generated from a transmission modulator (not shown) based on a control signal generated from control processor 18 via a digital-to-analog converter 22, in the transmission mode of the mobile station. Digital-to-analog converter 22 generates the control signal for controlling a gain of intermediate frequency amplifier 24. An output signal from intermediate frequency amplifier 24 is applied to an intermediate frequency amplifier 26. Since a gain of intermediate frequency amplifier 26 is controlled by an output of nonlinear filter 16, a transmission is signal output of intermediate frequency amplifier 26 is automatically controlled.
It is noted from the foregoing description that the prior art analog automatic gain control system includes the automatic gain control detector 12 for detecting an intensity of the received signal. However, it is difficult to form automatic gain control detector 12 at a broad variation range of a reception power level. Further, there is needed an additional means for converting an output level of automatic gain control detector 12 into a proper control voltage range for controlling the gain of intermediate frequency amplifier 6. Moreover, the prior art analog automatic gain control system needs comparator 14 for comparing the control signal output from control processor 18 with the output value of automatic gain control detector 12 in order to control the transmission power during the transmission and reception modes. However, as being composed of analog elements, comparator 14 is sensitive to noise so that a response time may be limitative. Therefore, it is not easy to accurately control the output of comparator 14. Besides, the prior art analog automatic gain control system needs digital-to-analog converter 20 for converting the digital signal output from control processor 18 into the analog value to supply the analog value to comparator 14. However, a low conversion speed of digital-to-analog converter 20 becomes a main cause that delays the response time of the automatic gain control system. In particular, in case that the signal that the mobile station has received is a burst signal, it is difficult to control the gain in order to maintain a constant received signal level through one data frame interval. Further, it is not easy to realize nonlinear filter 16 in the light of its features. In order for nonlinear filter 16 to control the gain of intermediate frequency amplifier 26, there is needed an additional circuit, i.e., a level converter for converting an output level of nonlinear filter 16 within a gain variable control voltage range of intermediate frequency amplifier 26. However, although the level converter is additionally used, it is still not easy to constitute nonlinear filter 16 in the light of its nonlinear output characteristic. It should be noted that all the problems mentioned above are mainly caused because the prior art system employs the analog automatic gain control method.